Section 73. No boy or girl under sixteen shall engage in any of the trades or occupations mentioned in section sixty-nine in any street or public place after eight o’clock in the evening or before six o’clock in the morning, nor, except in the case of a boy or girl granted an employment permit by the superintendent of schools when such superintendent determines that the welfare of such boy or girl will be better served through the granting of such permit, during the hours when the public schools in the city or town where such boy or girl resides, or the school which such boy or girl attends, are in session.